There are many advantages to using flexible printed circuits (FPC). They utilize the thinnest dielectric substrate available for electronic interconnection. FPC is known for its ability to reduce package size as well as package weight. FPC can significantly reduce the weight of an electronic package. This weight reduction makes flexible circuits extremely popular in the portable communication electronics industry. Another advantage of flexible printed circuits is the assembly costs. Reduction of assembly costs is achieved by simplifiing the assembly operations and, in addition, capable of testing the circuitry prior to committing it to assembly work. A properly designed FPC is an excellent means of simplifying the level of interconnection required in an electronic package.
Typically, a laminate of flexible printed circuit consists of a base layer of dielectric and a conductor layer of metal foil. An adhesive layer may be employed to bond the base layer to the conductor layer to form a laminate. Most laminates typically use polyester or polyimide film as flexible base material and use copper foil as conductive layer. On one hand polyimide has excellent electrical, mechanical, and chemical properties in addition to its high thermal and oxidative stability. On the other hand, most flexible printed circuits manufacturing involve a high temperature soldering process, therefore, a polyimide base laminate seems be the best choice today for flexible printed circuits applications.
Adhesive layer has a weak link in a laminate for flexible printed circuits. A so-called adhesiveless laminate can be superior to adhesived laminate in most important areas such as thinness, high temperature resistance, plated through hole processes, and dimensional stability. For application of flexible printed circuitry, thickness reduction is a major benefit brought by adhesiveless laminate and said adhesiveless laminate could also enhance the required dimensional stability and higher flexibility. More importantly, these circuits which have no epoxy adhesive layer could be thermal-cycled without compromissing their terminal integrity or dielectric degradation.
Adhesiveless polyimide laminate may be produced through casting of a polyimide resin onto a metal foil, so-called cast-on-foil, or by metallizing of a polyimide film, so-called seeded film. The process of metallizing of a polymer film is extremely complicated due to the chemical or temperature sensitive conditions of the metallizing processing. Furthermore, production of seeded film can not provide a rolled and annealled type copper clad laminate which as excellent flexure life and is comparable inexpensive of cost and has various choice of metal foil. Finally, the cast-on-foil process can easily be a roll-to-roll process.
However, the cast-on-foil process would encounter a problem, which is, the polyimide resin has poor adhesion to the metal foil. Many solutions in prior inventions tried to promote the adhesion between polyimide resin and metal, which are described as below:
A modified polyimide resin incorporating a siloxane component has been used to enhance the adhesion between polyimide resin and copper foil. However, siloxane incorporation may reduce the thermal properties of polyimide resin.
U.S. Pat. No. 5,200,474 disclosed a polyimide incorporating with bismaleimide which was modified by barbituric acid or the derivatives, used in casting onto copper foil directly. This invention, however, needs more processes to prepare a modified bismaleimide and to mix said modified bismaleimide with polyimide.
U.S. Pat. No. 5,372,891 disclosed a polyimide casting method comprising a polyimide upper layer and a polyimide-bismaleimide lower layer, which exhibits good adhesion characteristics with copper foil. However, this invention must prepare more compositions, and its processes involve more than one casting layer, which makes the production very complicate.
U.S. Pat. No. 5,290,909 disclosed a polyimide prepared by the polymerization reaction involving imidazole- or benzimidazole-containing amine monomers, which makes a polyimide/copper laminate without applying an adhesive layer. This invention, however, needs preparing a modified monomer from more complicated compounds.
The present invention, importantly, produces a polyimide resin by using more common monomers. No specially modified monomers are required in the present invention.
Also in present invention the polyimide resin which contains small amount of inorganic fillers exhibits a dramatic effect in promoting the bonding strength between polyimide and copper foil.